The present invention relates generally to a pilot valve and more specifically to a pilot valve that is selectively switchable between modes of operation and is adjustable in at least one mode of operation.
As is known in the art, pilot valves are used in a number of different industries, typically to control process conditions or parameters, by controlling the operation of a process control unit, such as a process control valve. Numerous industries utilize process control valves to control the rate of flow of liquid and to control the level of liquid in tanks and vessels. The process control valves in many cases are operated with a control signal, which may be, for example, a pneumatic signal transmitted from a pilot valve. In such a case, the liquid level in a tank or other vessel may be sensed with a float or other element which moves when the liquid reaches a certain level. Movement of the float will actuate a control device, such as a pilot valve, which will send a signal to the process control valve to open or close the process control valve to either allow flow of liquid from the vessel or to prevent flow therefrom.
In the oil and gas industry, liquid level controllers may be used to control the level of oil, water or other liquids in a tank or other vessel. Liquid level controllers may include a float, or displacer placed inside the tank. When the liquid level in the tank is such that it engages the displacer, the displacer will move as the level of the liquid changes. The changes in the liquid level are transmitted to a pilot valve which will signal the process control valve to either open or close in response to the changing liquid level in the tank which will either allow flow or stop flow from the tank.
The pilot valve may be a pneumatic pilot valve to which a supply gas is provided. The pilot valve will direct the supply gas to the process control valve depending upon the level of liquid in the tank and the process control valve will open or close depending on the signal received from the pilot valve. When the process control valve is a pressure to open the valve, the pilot valve will modulate output gas to the control valve which will open the control valve. Liquid level controllers may be referred to as direct and throttle in operation if an increase in the level in the tank results in a proportional increase in outlet pressure from the pilot valve to the process control valve. In such a case the process control valve may move from closed to partially open and ultimately to fully open or anywhere in between the fully open and fully closed positions since the gas pressure supplied to the process control valve from the pilot valve will vary.
When the pilot valve is in snap mode, the pilot valve provides a sudden increase in output pressure, or a sudden decrease to the process control valve so that the control valve will open fully, or close fully, almost immediately. For example, in a direct snap mode, when the level of liquid in the tank increases and moves the displacer to a predetermined level, there will be a sudden increase in output pressure from the pilot valve which will cause the pressure to open the process control valve to move almost immediately from the fully closed to a fully open position to discharge liquid from the tank. When the liquid level falls to a preselected lowermost level, a sudden decrease in output pressure from the pilot valve will occur, which will cause the process control valve to move from the fully open to the fully closed position.
U.S. Pat. No. 7,392,822, assigned to the assignee of the current application and incorporated herein by reference discloses a pilot valve that is selectively switchable between snap and throttle modes. While the pilot switch disclosed therein works well, there is a continuing need for pilot valves that are switchable between modes, and that are adjustable to provide for opening and closing of a process control valve over a range of liquid level changes.